asoiafbracketology : He is trying to prove that Joffrey is not Roberts son. Roberts seed is strong so Gendry looks exactly like Robert but Joffrey looks nothing like him. So Ned uses age dry Roberts actual son to determine that none of cerseis children are Roberts

lotr_strider : Can you pls DM me your profile picture? I love it😍

starkgaryen : @asoiafbracketology Ooh thank you 💕

gameofthrones__got : Because Gendry is Robert's son. And he looks exactly like him. While Joffrey doesn't look like Robert. Just as @asoiafbracketology said 'the seed is strong' so Robert's kids look exactly like him. Joffrey is different, he has golden hair and that helps Ned to understand that Joffrey is actually Jaime's son. Idk if I explained good😐😂